The present invention relates to the field of calenders, and more particularly to devices for controlling the diameter of rolls used in calenders or analagous machines.
Pressing a material between two calender rolls can change the physical characteristics of the material. For example, calendering paper changes its density, thickness and surface features. Thus, the calendering process is frequently used in the manufacture of paper and other sheet materials where it is often desirable to change the density, thickness or surface features of the material.
A common problem associated with calendering is the uneven thickness of the calendered material or "web." Localized variations in a variety of parameters affect the diameter of individual calender rolls and create variations in the spacing or "nip" between cooperating rolls. Variations in the nip across the width of a pair of calender rolls produces a web having non-uniform thickness. Thus, a more uniform web thickness could be obtained if the local diameters of the calender rolls could be controlled.
If a calender roll is made of a material that responds to changes in temperature, one may control local roll diameters by varying the temperature of selected cylindrical sections or "slices" of the roll. Previous devices have used this principle by directing jets of hot or cold air against slices of a rotating calender roll to control the local diameters of the roll.
Many of these previous devices blow jets of hot air from a hot air supply plenum against selected slices of the calender roll to increase the local diameters of the roll and thus decrease the local thickness of the web. Alternatively, when these devices blow jets of cold air from a separate supply plenum against slices of the calender roll, these slices contract. This decreases the local roll diameter and increases the local thickness of the web. Nozzles communicating with the interior of each plenum direct these jets of air against the calender roll. The nozzles are generally disposed at intervals corresponding to adjacent slices of the calender roll whose local diameters are to be controlled. Examples of such devices are shown in U.S. Pat. No. 4,114,528 to Walker and U.S. Pat. No. 3,770,578 to Spurrell.
In these previous devices, the heated air directed by the nozzles against the calender roll is lost to the surrounding atmosphere after it contacts the roll. Thus, these devices loose a relatively large amount of heat energy to their surroundings. In contrast, the apparatus of the present invention recirculates a substantial portion of the heated air after the air contacts the calendar roll. Thus, the device of the present invention is more energy efficient than many previously known calender roll control devices. This and other advantages of the present invention will become apparent in the description which follows.